In conventional magnet-type rodless cylinders, in general, a mechanism is utilized in that as pistons having inner magnets on the circumferential surfaces move due to internal pressure, a slide body having outer magnets magnetically coupled to the inner magnets moves being attracted by the motion of the inner magnets.
The magnitude of the attracting force is called “magnetic holding force” and is used as an index that represents the conveying ability of a magnet-type rodless cylinder.
FIG. 19 is a view schematically illustrating a general magnet-type rodless cylinder conventional design.
In FIG. 19, four outer magnets 102 of a slide body 101 on the outer side of a tube 100, and four inner magnets 104 of a piston 103 on the inner side of the tube 100 are arranged placing yokes 105 therebetween in a manner so that the same poles are opposed to each other in the axial direction. Further, the inner magnets 104 and the outer magnets 102 are arranged so that different magnet poles are opposed to each other.
Here, the magnetic holding force is defined as the force acting on the slide body in the axial direction 101 when the inner magnets 104 are deviated (displaced) in the axial direction relative to the slide body 101 (outer magnets 102) while applying a fluid pressure to the piston 103 without permitting the slide body 101 to move in the axial direction.
FIG. 4B is a diagram schematically illustrating the relationship between the amount of deviation (amount of displacement) of the inner magnets 104 and the magnetic holding force. In a static state where no fluid pressure is applied as shown in FIG. 4B, i.e., in a state where four inner magnets 104 and four outer magnets 102 are at positions, which are in alignment with each other in the radial direction without being deviated in the axial direction, the magnetic holding force becomes zero at point A. Magnetic holding force increases as the deviation between the inner magnets 104 and the outer magnets 102 in the axial direction increases, and becomes a maximum value Max (point B) when the deviation is about one-half of the pitch L of the magnets 102, 104 in the axial direction.
JP-UM-A-4-113305 discloses an art for flattening the cross sections of the cylinder tube and of the piston in the radial direction, in order to decrease the size of the device, by decreasing the thickness of the cylinder or to increase the cylinder thrust.
JP-A-4-357310 discloses a magnet-type rodless cylinder in which the cylinder tube and piston are formed in an oblong circular shape, in an oval shape or in a symmetrical pear shape in the radial direction in cross section.
Further, Japanese Utility Model Registration No. 2514499 discloses the arrangement of two magnet-type rodless cylinders in parallel having a slider, which strides the pair of cylinders.
JP-B-3-81009 discloses a rodless cylinder of the slit tube-type, the cylinder tube having two cylinder holes. The pistons are arranged in the cylinder holes, and are mechanically coupled to a slide body on the outer side of the tube through slits sealed with bands.
Further, U.S. Pat. No. 3,893,378 discloses a rodless cylinder of the slit tube-type, the tube having a rectangular outer shape in cross section and a cylinder hole having a quadrilateral shape.
JP-A-9-217708 discloses a cylinder of the rod-type, the cylinder tube having two cylinder holes.
British Patent No. 470088 discloses a rodless cylinder of the slit tube-type, the cylinder tube of a non-circular outer shape having three cylinder holes.
JP-UM-B-4-010407 discloses a magnet-type rodless cylinder in which a notch is formed in a slide body for passing a mounting member.
JP-B-3-81009, U.S. Pat. No. 3,893,378 and British Patent No. 470088 are related to the technologies of slit tube-type rodless cylinders, while JP-A-9-217708 is related to the technology of a rodless cylinder. These patent documents are referred to in this specification as general background art in the field of fluid pressure cylinders.
In general magnet-type rodless cylinders utilized in the field, the exactly circular cylindrical tube undergoes uniform deformation if the internal pressure of fluid is applied thereto. In tubes having a flat and noncircular outer shape as taught in JP-UM-A-4-113305 and JP-A-4-357310, since only one cylinder hole of a noncircular shape is formed, the tube is not uniformly deformed when the internal pressure of fluid is applied thereto, and therefore, maximum stress and maximum deflection are substantial.
To avoid this, the thickness of the tube must be greatly increased, and thereby a problem occurs in that the magnet-type rodless cylinder will not work unless the magnetic coupling force is also increased by several fold. Previously, therefore, two exactly circular cylindrical tubes were arranged in parallel as taught in the Japanese Utility Model Registration No. 2514499. However, the structure required for arranging a plurality of tubes in parallel as taught in Japanese Utility Model Registration No. 2514499 involves a cumbersome assembly operation, as well as increased space for installation, which are not desirable.
When the general magnet-type rodless cylinder is in a static state, the inner magnets 104 and outer magnets 102 are in alignment attracting each other in the radial direction in FIG. 19 without being deviated in the axial direction. Therefore, magnetic holding force is zero.
Therefore, if the piston 103 is attempted to be moved in this state, the motion is not smooth and a stick-slip phenomenon at the start of the slide body 101 occurs, since the outer magnets 102 are not attracted to the inner magnets 104 until the above “deviation” occurs.
The above problem occurs even with the device having a tube of a noncircular outer shape as taught in JP-UM-A-4-113305 and JP-A-4-357310. The same problem occurs between the cylindrical tubes and the slide body even with the constitution taught in Japanese Utility Model Registration No. 2514499 in that two of an exactly circular cylindrical tubes are arranged in parallel maintaining a relatively long distance.